Why does my iPhone 6s have an echo?

IOS 9.2.1, restarted daily. Not using a headset but has a persistent echo during all calls, in all locations. Echo is present only when no headset is used. Please tell me how to make it stop! Charge is great.

I recently got an iPhone SE. It echo'd terribly, so we returned it. Long wait, as it was out of stock "everywhere". Finally got a new one again today, and it also had an echo when I test-called my husband. Later I spoke to my son and realized there wasn't an echo so by process of elimination the problem had to do with hubby's Huawei. I got him to turn his phone off and back on and voila! problem solved. Whoda thunk it.